The invention relates to a method of producing accretion-free holes with a diameter of less than 1.5 mm in circuit boards made of plastic, preferably circuit boards consisting of a plurality of layers, by means of a hard metal drill bit exhibiting a cylindrical chucking shank and a fluted twist bit.
With materials which, when drilled, do not produce continuous chips but break down into boring powder, there is a danger of the boring powder or dust being pressed against the wall of the hole by the side cutter on the bit and forming a solid film thereon through friction. These accretions are very hard to remove. When drilling multi-layer circuit boards the accretions in the contact holes lead to poor contacts between the separate internal layers. Therefore it has already been proposed in German OS No. 1,704,296, for example, that the boring dust be removed quickly using a jet of compressed air by means of which the boring dust is supposed to be blown out of the contact holes during drilling or when drilling has been completed. However this approach did not produce the desired success. When drilling such contact holes, which have a maximum diameter of 1.5 mm in circuit boards made of synthetic resin, high cutting speeds are used; consequently one has to employ twist drills made of carbide material, of so-called hard metal, to drill the contact holes. To make it easier for the boring dust to flow away during drilling, use is made of so-called "headed" drills in which the fluted twist sections are set back some 0.6 mm behind the tip. This arrangement is supposed to largely eliminate accretion of the boring dust on the wall of the hole due to friction; however even this arrangement can only reduce accretion but not prevent it.
The tip of the drill is constituted by a transverse cutting edge to which the main cutters are connected on either side of the center line of the drill. These main cutters run up to the cutting edges of the twist drill which the spiral side cutters terminating the fluting are adjacent to. The transverse cutting edge is subjected to the greatest load as it cuts into the material less but exerts a rubbing action on the material to be drilled.